Long-term trends and clinical and procedural determinants of system impedance in subcutaneous implantable cardioverter-defibrillators

System impedance is a key surrogate for defibrillation efficacy in subcutaneous implantable cardioverter-defibrillator (S-ICD) systems. S-ICDs feature an automatic low-voltage impedance (LVI) measurement function, which assesses impedance over time using subthreshold pulse delivery. However, the long-term trend of system impedance in S-ICD recipients remains uncharacterized.

This study aimed to describe the long-term trends of LVI in S-ICD patients and identify potential clinical and procedural determinants.

We analyzed data from 1,226 patients who underwent de novo S-ICD implantation across 15 Italian centers. Baseline characteristics, implantation variables, and LVI data were collected over a median follow-up of 37 months [IQR: 20–57]. Post-implantation radiographs were reviewed to calculate the PRAETORIAN score.

The mean LVI increased significantly during the first three months post-implantation (from 59±14 to 76±16 Ω, P < 0.001) before stabilizing (77±17 Ω, P = 0.231). Impedance was largely unaffected by patient clinical characteristics, except for higher values observed in overweight or obese individuals, as well as in cases of subcutaneous device positioning and the three-incision lead deployment technique. Higher impedance values were also associated with significant sub-coil fat and a PRAETORIAN score ≥90.

After an initial increase, system impedance stabilizes by the third month post-implantation, suggesting completion of the healing and tissue maturation phase. Implantation techniques that minimize fat interposition between the device and chest wall result in lower long-term impedance values.